1. Field of the Invention
The invention relates generally to the field of data processing and more particularly to servo systems for centering a disc drive's read/write head on a track.
2. Brief Description of the Prior Art
In disc drives having a dedicated servo surface and a plurality of data surfaces wherein the data heads are mounted on the same carriage as the servo head in vertical alignment therewith, it has been proposed that any radial offset of a data head from a servo head be electronically measured and later compensated for when positioning a data head over a data track. See IBM Technical Disclosure Bulletin, Vol. 17, No. 6, Page 1781, November 1974.
In the IBM technical disclosure, one or more servo tracks are written on a data surface. During a calibration procedure the servo head is served to a corresponding servo track on the servo surface. A voltage representative of the radial offset of the data head from its servo track center (null) is then developed in the same manner as the servo head position error signal is developed. The resulting error signal is converted from an analog voltage into a digital value and stored in a digital memory. Thereafter, when positioning the data head over the data track, the servo head is positioned off center from its own servo track center (null) by the theoretical amount of the radial offset measured in the calibration procedure such that the data head is positioned directly over its data track center. This is accomplished by retrieving the digital value from memory, converting it back into a voltage and injecting this voltage into the servo loop of the servo head.
A problem with this approach is that the analog voltage developed by the position error measurement system from the data head may not be representative of the same offset as a similar voltage developed by the servo head position error measurement system. That is, the same voltage from the two positioned error measurement systems might actually be representative of two different offsets. The cause of this might be due to many factors including differing gains in the two sets of circuits. But as a result, injection of the offset voltage measured from the data head into the servo head servo loop may produce a different offset from that desired. The data head will then not be centered on its data track as desired.